Process for producing a composite electrode



United States Patent O 3,236,696 PROCESS FOR PRODUCING A COMPOSITE ELECTRODE Henri Georges Andr, Montrnorency, France, assignor to Yardney International Corp., New York, N.Y., a corporation of New York No Drawing. Filed Sept. 21, 1961, Ser. No. 139,598 Claims priority, application France, Sept. 22, 1960, 839,246, Patent 1,275,025 6 Claims. (Cl. 136126) This invention relates to an improvement in alkaline accumulators, more particularly in those utilizing a negative electrode made of zinc, cadmium or iron.

In many accumulators of the silver-zinc type the negative electrode is provided with a copper support which plays an important part in the operation of this electrochemical system. Thus, in an accumulator using sheet zinc as the negative active material, a copper support greatly facilitates the flow of electric current and at the same time is fully compatible with zinc with which it has no tendency to develop any objectionable local couples, as a result of its property of becoming coated with a thin layer of the latter metal.

Experience has shown, however, that in a great many cases local couples do develop by reason of electrode impurities and lack of surface uniformity. While such couples can sometimes be eliminated by resorting to highpurity metal, this is a costly method and one that is not always fully effective. Local couples are particularly likely to develop when the electrode surface is rough, this being desirable from the electrochemical viewpoint. In many cases their presence is particularly objectionable because of the gas that they generate.

In many types of primary cells, using zinc sheet negative electrodes, the elimination of local couples is achieved 3 by a direct amalgamation of the zinc sheet. This method, however, is not applicable to secondary cells or accumulators because of violence of the reaction which takes place between zinc and mercury when these two metals are brought in contact. As a result of this reaction, the zinc loses its mechanical cohesion and becomes brittle which makes it unsuitable for subsequent handling and shaping.

It is accordingly an object of this invention to provide a method of amalgamating a zinc electrode in such manner that the amalgamated electrode retains its malleability for a sufficiently long time to permit subsequent handling in assembly.

It is a further object of this invention to provide an amalgamated zinc electrode which, when used in conjunction with a copper support, will not develop gasgenerating local couples while possessing an optimum structure from the electrochemical point of view.

Other and more detailed objects will be apparent from the following description.

The objects of this invention are achieved by an amalgamation process which can best be described as a two-step process. According to the invention it is the copper support rather than the zinc sheet which is subjected to direct amalgamation by dipping it into a mercury-containing solution. Following this operation, the amalgamated copper support, usually in the form of a thin copper sheet, is juxtaposed on a zinc sheet of the same dimensions. As a result of this juxtaposition, the mercury present on the surface of the copper support is brought into contact with the zinc surface, thereby causing the amalgamation of the latter.

It was found that while a direct amalgamation of a sheet of zinc by mercury leads to a violent reaction caus ing a rapid deterioration of the zinc structure, making it brittle and unsuitable for further handling and folding,

the amalgamation of copper is a substantially less violent process which does not deprive the metal of its mechanical cohesion. When a sheet of copper is treated with a mercury-containing compound, the mercury diffuses slowly into the copper so that the concentration of mercury on the copper surface decreases with time.

It was further found that surprisingly the amalgamation of copper does not interfere with its polarization properties with respect to zinc, that zinc sheets or the like when placed in contact with amalgamated copper will become amalgamated gradually and slowly, that the zinc amalgamated in this indirect manner will not become brittle and difficult to handle and that its electrochemical properties will be superior to those of the Zinc amalgamated by a conventional method.

In amalgamating the copper support in accordance with the present invention, it is advisable to use an excess of mercury. In this manner a source of mercury is available as a reserve for an automatic regeneration of those areas of active material which have lost their mercury as a result of cell operation. In amalgamating the copper support in accordance with the invention, the amount of mercury supplied to the copper can be measured to exactly meet the requirements.

Although, as mentioned above, the amount of mercury contained in the copper support may vary considerably, it has been found that particularly good results are obtained with supports, e.g. of copper, which are amalgamated to the extent that they contain from .05% to 5% by weight of mercury based on the total weight of the support.

Since the concentration of mercury on the copper surface decreases with time, the action of mercury on Zinc can easily be controlled by varying the length of time allowed to elapse between the instant when the copper support is amalgamated and the instant when it is brought in contact with the zinc.

An alkaline accumulator in which the negative electrode has been amalgamated in accordance with the invention will be free from local couples and as a result will not generate any gas in normal operation. As a further result it will not consume water and will not require any replenishment over a large number of working cycles. This in turn will prevent the separator from deteriorating and minimize the occurrence of short-circuits and similar failures.

Furthermore, the stability of the electrochemical system and the constancy of a homogeneous distribution of mercury particles within the electrode, that will be achieved by amalgamating the electrode in accordance with the invention, will result in an accumulator having a capacity that will not be adversely affected by cycling and one that will afford a higher operational safety, which will make such an accumulator particularly suitable in applications where these considerations are a factor.

The invention is not restricted to alkaline zinc accumulators but also applies to cadmiumor iron-type secondary cells.

According to this invention, the amalgamation of the copper support in the form of a sheet or foil is preferably carried out by dipping the support into an aqueous solution of mercury bichloride to which hydrochloric acid HgCl +HCl+Cu CuClfi-HCl-j-Hg which can also be written as follows:

HgCl +Cu- CuCl +H g It was found that the copper sheets or foils treated in the above manner retain their malleability long enough to permit them to be easily assembled with the elements with which they will constitute an electrode. The assembly operation, however, must be carried out within a reasonable length of time, i.e., before the onset of embrittle-ment, so as to take full advantage of the temporarily malleable state of the amalgamated metal.

In accordance with the invention, the elements to be assembled with the amalgamated copper support are preferably prepared beforehand and the assembly operation is carried out immediately following the amalgamation. The zinc sheets are amalgamated individually by bringing each sheet into contact with an amalgamated copper support. However, if desired, the process may be carried out continuously with zinc strip being fed in a permanent manner so as to fit into a mechanized mass-production scheme.

It has been further found that, upon a storing of the amalgamated copper in water, its malleability is retained for a longer period of time. An amalgamated copper support washed and stored in water will remain malleable for more than 24 hours, thereby allowing ample time for further assembly.

An accumulator incorporating a negative electrode amalgamated in accordance with this invention will not only be free from gas evolution under normal operating conditions but also be capable of withstanding severe treatment without damage. For example, any gas generated in the event of an overcharge will be reabsorbed by the negative electrode at the end of the following discharge, thereby making it possible to repeat overcharges without any ill effect on accumulator performance.

Because no gas is generated under normal operating conditions in an accumulator built in accordance with the invention, it can be enclosed in a hermetically sealed container without a risk of explosion. Hermetically sealed accumulators are valuable in many applications.

The following example illustrates a method of amalgamating a copper support in accordance with the invention. It is understood, however, that the invention is not limited thereto.

Example A solution having the following composition was prepared:

Bichloride of mercury grams Pure water cc 100 Hydrochloric acid, density 1.4 cc 50 Copper-sheet supports were immersed for 1 to 3 minutes in this solution, depending on their thickness. The amalgamated copper supports were washed in pure water, then dried in air. This support was then used to form a zinc electrode having a zinc sheet welded to each side of the amalgamated copper support. This may serve as a negative electrode in an accumulator having a silver positive electrode prepared in accordance with U8. Patent No. 2,818,462. In this accumulator the electrolyte may be a 44% aqueous solution of KOH. As a separator a semipermeable membrane consisting of 5 sheets of regenerated cellulose can be employed.

In a battery made in this fashion a hydrostatic valve either of the mercury type or of the paraffin-oil type will serve to prevent the development of excess pressures in high-rate cells and to prevent atmospheric air from penetrating into the battery.

Whereas the invention has been described with reference to specific forms thereof, it will be understood that many changes and modifications may be made without departing from the spirit of this invention as defined in the appended claims.

What is claimed is:

'1. A process for producing a composite electrode wherein an active metal selected from the group which consists of zinc, cadmium and iron forms an amalgamated layer on a copper sheet, comprising the steps of amalgamating a copper sheet, placing a substantially coextensive sheet of said active metal in contact with the amalgamated copper sheet before the onset of embrittlement thereof and allowing mercury from said copper sheet to diffuse into said sheet of active metal.

2. A process for producing a composite electrode with an amalgamated zinc layer on a copper sheet, comprising the steps of amalgamating a copper sheet, placing a substantially coextensive zinc sheet in contact with the amalgamated copper sheet before the onset of embrittlement thereof and allowing mercury from said copper sheet to diffuse into said zinc sheet.

3. A process as defined in claim 2 wherein said zinc sheet is brought into contact with said copper sheet immediately upon the amalgamation of the latter.

4. A process as defined in claim 2 wherein the amalgamated copper sheet is soaked in water for a prolonged period prior to being brought into contact with said zinc sheet.

5. A process for producing a composite electrode wherein an active metal selected from the group which consists of zinc, cadmium and iron forms an amalgamated layer on a copper sheet, comprising the steps of amalgamating a copper sheet by immersing it in an aqueous solution of bichloride of mercury highly acidified with hydrochloric acid, placing a substantially coextensive sheet of said active metal in contact with the amalgamated copper sheet before onset of embrittlement thereof and allowing mercury from said copper sheet to diffuse into said sheet of active metal.

6. A process for producing a composite electrode with an amalgamated zinc layer on a copper sheet, comprising the steps of amalgamating a copper sheet by immersing it in an aqueous solution of bichloride of mercury highly acidified with hydrochloric acid, placing a substantially coextensive zinc sheet in contact with the amalgamated copper sheet before the onset of embrittlement thereof and allowing mercury from said copper sheet to diffuse into said zinc sheet.

References Cited by the Examiner UNITED STATES PATENTS 600,693 3/1898 Julien 1363 1.1 621,523 3/1899 Dujardin 1363 1.1 945,243 1/ 1910 Morrison 136-3 1.1

FOREIGN PATENTS 7,259 1896 Great Britain. 7,259 of 1896 Great Britain. 276,797 9/ 1927 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner.

JOHN MACK, MURRAY TILLMAN, Examiners. 

1. A PROCESS FOR PRODUCING A COMPOSITE ELECTRODE WHEREIN AN ACTIVE METAL SELECTED FROM THE GROUP WHICH CONSISTS OF ZINC, CADMIUM AND IRON FORMS AN AMALGAMATED LAYER ON A COPPER SHEET, COMPRISING THE STEPS OF AMALGAMATING A COPPER SHEET, PLACING A SUBSTANTIALLY COEXTENSIVE SHEET OF SAID ACTIVE METAL IN CONTACT WITH THE AMALGAMATED COPPER SHEET BEFORE THE ONSET OF EMBRITTLEMENT THEREOF AND ALLOWING MERCURY FROM SAID COPPER SHEET TO DIFFUSE INTO SAID SHEET OF ACTIVE METAL. 